HIV-associated neurocognitive disorders (HAND) for both the young and aging population remain a huge and growing concern even in the presence of combination antiretroviral therapy (cART). Although viral replication is inhibited by cART, vira proteins such as TAT and gp120 are still produced within the nervous system which are neurotoxic and stimulate inflammation (1, 2). HIV induced neuroinflammation is thought to be a major contributing factor in the cognitive decline exhibited by roughly 50% of HIV-positive individuals (3, 4), but more information on the molecular and cellular mechanisms relevant to this persistent cognitive decline still needs to be addressed. Platelet activating factor (PAF) is n inflammatory mediator that can also alter neurotransmission (5-8). It is present at chronically high levels in patients with HAND as compared to HIV positive individuals without cognitive impairments (9). PAF receptor (PAFR) antagonism has been shown to be neuroprotective in mouse models of HAND and other neuroinflammatory diseases (10-17). Thus, PAF may be a major contributor to the neuronal injury associated with neuroinflammation and HAND because of its ability to modulate synaptic strength. Yet, there is much about the mechanisms of PAF signaling at the synapse that we do not understand including whether it occurs primarily in the presynaptic or postsynaptic compartment or both. I hypothesize that chronic PAF exposure increases neurotransmission leading to neuronal injury by acting on both the pre- and postsynaptic compartments. Enhanced neurotransmitter release and enhanced signal reception places synapses in a high activity state where they are vulnerable to excitotoxic injury and energy failure even in the presence of physiologic levels of stimulation. I will test this hypothess using both in vitro and in vivo techniques. In Aim 1, I will first determining the precise localizaion of the PAFR within the presynaptic and/or postsynaptic compartments. Then I will investigate PAF induced changes to synaptic vesicle organization and the proteins that regulate synaptic vesicle release within the presynaptic compartment. In Aim 2, I will test whether PAF signaling within the postsynaptic compartment is necessary or sufficient for neuronal injury. I will further investigate whether PAF signaling alters NMDA and AMPA receptor trafficking or activation state that would change the signal strength received by the postsynaptic cell. In Aim 3, I will determine the acute and long-term effects of PAF- induced synaptic injury in vivo and in a model of HAND using cutting-edge techniques including intravital multiphoton microscopy using cortical windows and optogenetics. Results from these studies should validate the PAFR signaling pathway as a viable target for the treatment of HAND and other neuroinflammatory diseases as well as increase our understanding of the molecular mechanisms leading to the cognitive decline in HAND patients.